1. Field of the Invention
The present invention relates generally to an optical filter device, and more particularly to an optical filter device employing electrically switchable holograms.
2. Description of the Relevant Art
Image display systems often employ a display screen for projecting an image. The display screen typically displays a sequence of monochrome images that are illuminated in succession by red, blue, and green light. The illuminated images are then projected for display to a viewer.
The display screen switches from one monochrome image to the next very rapidly so that a sequence of three consecutive monochrome images illuminated by red, blue, and green light, respectively, are projected and effectively eye integrated by a viewer to create a full-color image. The successive illumination of the display screen by red, blue, and green light can be achieved by employing a white-light source and a rotating color wheel (often utilizing transmissive dielectric red, blue, and green filters). These rotating color wheels are prone to mechanical problems. Additionally, rotating wheels tend to be large and noisy in operation.
The present invention relates to a solid state filter used in sequentially illuminating an image display, directly or indirectly, with first, second, and third bandwidth light. The solid state filter includes at least one hologram that is switchable between active and inactive states. While in the active state, the at least one switchable hologram diffracts a first bandwidth light. In contrast, the switchable hologram transmits the first bandwidth light without substantial alteration when operating in the inactive state. In one embodiment, the diffracted first bandwidth light is used to illuminate a monochrome image presented on a display device. In another embodiment, the transmitted first bandwidth light is used to illuminate the monochrome image presented on the image display.
In one embodiment, the solid state filter may be coupled to a filter control circuit. More particularly, the filter control circuit is coupled to the at least one switchable hologram of the solid state filter. The control circuit is configured to selectively couple a voltage source to the at least one switchable hologram. The at least one switchable hologram is configured to operate in the active state when decoupled from the voltage source. In contrast, the at least one switchable hologram is configured to operate in the inactive state when coupled to the voltage source.
In another embodiment of the present invention, a solid-state filter is provided which includes a first group of first, second, and third holographic optical elements electrically switchable between active and inactive states, and a second group of first, second, and third holographic optical elements electrically switchable between active and inactive states. Each of the holographic optical elements includes front and back oppositely facing surfaces. Each of the first holographic optical elements diffracts first bandwidth light incident on the front surface thereof when operating in the active state. Diffracted first bandwidth light emerges from the back surface of the first holographic optical element. In contrast, each of the first holographic optical elements transmits first bandwidth light incident on the front surface thereof without substantial alteration when operating in the inactive state. First bandwidth light transmitted by each of the first holographic optical elements emerges from the back surface thereof. Each of the second holographic optical elements diffracts second bandwidth light incident on the front surface thereof when operating in the active state. Diffracted second bandwidth light emerges from the back surface of the second holographic optical elements. In contrast, each of the second holographic optical elements transmits second bandwidth light incident on the front surface thereof without substantial alteration when operating in the inactive state. Transmitted second bandwidth light emerges from the back surface of the second holographic optical elements. Each of the third holographic optical elements diffracts third bandwidth light incident on the front surface thereof when operating in the active state. Diffracted third bandwidth light emerges from the back surface of the third holographic optical elements. In contrast, each of the third holographic optical elements transmits third bandwidth light incident on the front surface thereof without substantial alteration when operating in the active state. This transmitted third bandwidth light emerges from the back surface of the third holographic optical elements. In one embodiment, a polarization rotation device is positioned between the first and second groups of holographic optical elements. The polarization rotation device operates to rotate the plane of polarization of light transmitted therethrough.